Ninety-three per cent of homeowners with house insurance have left it up to their insurer to decide the level of cover they have, and that means $167 billion of property is uninsured.

This follows the shift by most insurers from total replacement policies to sum-insured policies where homeowners must decide what to insure their home for, and bear the risk of a shortfall if it costs more to rebuild.

Now national quantity surveying group Construction Cost Consultants (CCC), which has done 25,000 insurance reports in the last three years, has crunched the numbers on the dollar value of the underinsurance.

It says in the event of a full rebuild, relying on default sums means that most homeowners will be at least a quarter short of the rebuild cost and some will even be up to 50 per cent shy.

"Based on that 25 per cent estimate, the number of private dwellings [1.765 million] and the median national house price [$407,525], we estimate that $167b worth of residential property, at minimum, is currently not insured for rebuild," said Gary Caulfield, CCC's managing director for residential properties.

It estimates the total value of housing in New Zealand is at least $717b, Caulfield said.

Had sum insured been in place before the Christchurch earthquakes it would have been a disaster for homeowners, he said.

Homeowners have the choice of paying for a consultant to accurately estimate their rebuild costs or using online calculators provided by insurers. But CCC says in almost every case it has seen, the online calculators recommend a sum insured higher than the default sum.

The irony is, Caulfield says, is that the last $100,000 of cover on a sum-assured policy is relatively cheap, especially on larger houses. "Once you get above $400,000 to $500,000, your next $100,000 costs about $100 or less [a year]."